1. Technical Field
The present invention relates to an LED driving circuit, an illuminating device, and an electro-optical device.
2. Related Art
In a backlight of an electro-optical device, such as a liquid crystal display device, instead of a conventional cold cathode fluorescent lamp type, a method using a plurality of LEDs having different emission peak wavelengths, such as red (R), green (G), and blue (B) LEDs, has drawn attention. This method can realize a higher degree of color purity and higher color reproducibility than the cold cathode fluorescent lamp.
The brightness of the LED can be adjusted by a driving voltage supplied to the LED. It is possible to accurately adjust the color of light emitted from the LED and thus to realize high color reproducibility by minutely adjusting the brightness of each LED. That is, it is important to minutely adjust the driving power supplied to the LED in order to improve the display quality of an electro-optical device.
For example, JP-A-2003-215534 discloses a method of improving visibility of a liquid crystal display device and of reducing power consumption by adjusting the driving power supplied to an LED that is used for a backlight of the liquid crystal display device, corresponding to the surrounding brightness.
An LED driving circuit disclosed in JP-A-2003-215534 includes a voltage dividing circuit that divides an input voltage by using a plurality of resistors connected in series to each other and a selection circuit that, when a control signal is supplied, selects one of the divided voltages on the basis of the supplied control signal and outputs the selected voltage. According to this LED driving circuit, since a voltage is selected from a plurality of voltages on the basis of the control signal, it is possible to minutely adjust a driving voltage to be supplied to LEDs so as to correspond to the surrounding brightness.
However, in the above-mentioned LED driving circuit having the voltage dividing circuit, it is necessary to increase the number of resistors constituting the voltage dividing circuit, in order to minutely adjust an output voltage so as to adjust the color of light emitted from the LED.
For example, it is assumed that voltages of 0 V and 4 V are supplied to both ends of the voltage dividing circuit. In this case, twenty resistors are needed in order to output voltages at voltage intervals of 200 mV in the range of 0 V to 4 V. In addition, forty resistors are needed in order to output voltages at voltage intervals of 100 mV in the above-mentioned range.
Therefore, the above-mentioned LED driving circuit has a problem in that larger resistors are needed in order to minutely adjust an output voltage better, which results in an increase in the size of an LED driving circuit.